Numerical simulations of depleted (CH4) and cushion (CO2) gases impacts on H2 withdrawal and CO2 storage efficiencies in a depleted gas reservoir

Abstract

Hydrogen (H2) storage in depleted gas (mainly CH4) reservoirs associated with carbon dioxide (CO2) injection as cushion gas is one promising strategy to promote the energy transition and decarbonization. However, it is still unknown about how depletion time, gas volume ratio of H2, CH4 and CO2 (VRH2:CH4/CO2), and injection/withdrawal rate will influence the performances of H2 withdrawal and CO2 storage in depleted gas reservoirs. Therefore herein, numerical simulations were performed to examine these influences systematically. The following results are demonstrated: 1) in the presence of H2 and CH4, as the VRH2:CH4 decreases from 100 %: 0 to 50 %: 50 %, H2 withdrawal factor (WF-H2) and purity (WP-H2) firstly increase and then decrease, with the maximumWF-H2 = 42 % and the minimumWP-H2 = 51 %-91 % occurring at 60 %: 40 %; 2) in the presence of H2 and CO2, as VRH2:CO2 increases from 50 %: 50 % to 75 %: 25 %, WF-H2 and WP-H2 continuously decreases with the maximumWF-H2 = 38 % and the maximum WP-H2 = 29 %-96 % occurring atVRH2:CO2 = 75 %: 25 %, while CO2 storage factor (SF-CO2) continuously decreases from 59 % to 2 %; 3) in the simultaneous presence of H2, CH4 and CO2, synergistic impacts of CH4 and CO2 result in 4 % WF-H2 and 60 % SF-CO2 enhancements, respectively, occurring at VRH2:CH4 = 90 %: 10 %, and VRH2:CO2 = 75 %: 25 %; 4) in the case of H2 and CH4, a smaller H2 withdrawal rate (WR-H2) results in a lower WF-H2, but a higher WP-H2. These insights provide crucial guidance for the implementation of temporary H2 and permanent CO2 geo-storage in depleted gas reservoirs.